The invention relates to NMR imaging, and more particularly relates to the production of NMR images which are gated in accordance with body motion of a living patient. In its most immediate sense, the invention relates to retrospective gating of NMR images without direct monitoring of a patient's body processes.
Diagnosticians have long used gated images when diagnosing moving body parts. For example, when conducting a heart study, it is advantageous to isolate images which relate to one or more predetermined phases of the cardiac cycle. Thus, a physician may wish to view a series of images which all relate to the patient's heart at various points in his ECG.
One way in which such gated images have been produced for cardiac imaging applications is to connect appropriate electrodes to the patient and to monitor the patient's ECG while the patient is undergoing an NMR study. This technique has proved unsatisfactory. This is because the intense magnetic fields and pulsed magnetic field gradients used in NMR often interfere with the patient's ECG signal. As a result, in an attempt to find electrode locations which produce satisfactory ECG signals, it is frequently necessary to repeatedly start an NMR study, stop the study when the ECG signal proves unsatisfactory, withdraw the patient from the NMR unit, and reposition the electrodes. All this is time-consuming, and therefore expensive (considering the high costs of operating an NMR unit).
It is also known to gate NMR data using a priori assumptions about periodic motion which is used to drive the gating process. For example, European Patent Application No. 0 182 265 teaches the concept of empirically determining an average breathing rate using a constant phase-encoding gradient and then using that average rate to gate a subsequent imaging study. This is unsatisfactory because body processes are not exactly regular.
It is additionally known to gate NMR data retrospectively. This is illustrated by U.S. Pat. No. 4,710,717, which teaches the concept of varying the phase-encoding gradient in accordance with the cardiac cycle and interpolating the NMR data as necessary to produce appropriate images. This is not generally applicable because it is necessary to sense the ECG directly.
In none of these known techniques is it possible to simply and accurately gate NMR data retrospectively in accordance with any arbitrarily-selected body motion without undue consumption of expensive study time.